During initial power on of a power supply or a rectifier, a large input current will charge the bulk capacitor bank of the power supply or rectifier, since the capacitor voltage was at zero before power turn on. This current is a function of the input peak voltage and the input source impedance and is generally referred to as an inrush current. Inrush current must be limited to a certain level. Without such limitation, the inrush current may trip the input protective devices such as, for example, circuit breakers and fuses. The inrush current also generates unwanted noise and affects equipment connected across the power mains.
Inrush resistors are typically used to limit the inrush current level and are connected between the input AC line and the bulk capacitor bank. The inrush resistor must, however, be removed from the circuit during the normal operation of the power supply to achieve high efficiency. A DC power driven inrush AC relay is typically used in the power supply and rectifier to bypass the inrush resistor during normal operation of the power supply or rectifier. Where the inrush current limiting resistor and inrush relay combination is located between the power supply diode bridge rectifier and the bulk filter capacitor bank, this current limiting scheme can not limit the inrush current into the "X" capacitor, across the AC lines, of the input EMI filter. In order to overcome this shortcoming, the inrush resistor and the inrush relay combination is relocated before the input EMI filter. However, where the inrush resistor and inrush relay are located before the EMI filter section, power supply EMI deficiencies result. In the construction of the electromechanical relay, there exists parasitic capacitors between the relay DC coil and the AC power contacts of the relay. Since the relay coil is driven by a low voltage onboard bias via two long leads, the high frequency switching noise (differential mode and commonmode noise) is conductively coupled to the AC input without suppressing prior to the EMI filter. The two long control leads of the relay coil also become a coupling channel to transmit noise directly to the noise receptor circuit, including the input EMI measuring equipment.
A need has thus arisen for an EMI filter to suppress noise and to minimize noise transmission and coupling introduced from a power supply or rectifier into the AC source due to use of an inrush relay before the input EMI filter section.